Transmission control in two-way signaling system



y 1945- K.}G. VAN WYNEN I 2,379,768

TRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEM Filed Oct. 1, 1942 3 Sheets-Sheet 1 v M02 I H V WL Tim H 7 EL/051.4, FIG 2 [54 i 'V,

E WL 7; E4

R/Vn' H I //A 4MP FIG. 3 5K Mi He 4 f L V 4m "Q ATV 4MP M/VEN/TOR K a VAN wmaw V AZ TORNEV July 3, 1945. K. e. VAN WYNEN 2,379,768

TRANSMISSION CONTROL IN TWO-.WAY SIGNALING SYSTEM I Filed 001:. m, '1942 3 Sheets-Sheet 2 we 4 I I V v I 4MP.

A fro/war Patented July 3, 3945 TRANSMISSION CONTROL IN TWO-WAY glGNALING SYSTEM Kenneth G. Van Wynen, Ramsey, N. .L. asslgnor to Bell Telephone Laboratories, Incorporated, New York. N. Y., a corporation of New York Application October 1, 1942, Serial No. 460,374

8 Claims. (Cl. 178-44) This invention relates to two-way telephone systems, and particularly to circuits for directionally controlling signal transmission in such systems while suppressing echoes and preventing slnglllg.

An object of the invention is to operation of such circuits.

A more specific object is to improve the singing margin and reduce echoes in two-way telephone repeater circuits.

These objects are attained in accordance with improve the 'the invention by the application of thermistors to a two-way telephone repeater in such manner as to increase the loss in the singing path thereby enabling a greater reduction in the amount of echo for a given amount of amplification, or increased amplification fora given amount of echo to be attained. This 'is accomplished in accordance with one embodiment of the invention by using a portion of the signal output energy of the amplifier in each repeating path to heat a thermistor element connectedin the input of the amplifier in the oppositely directed repeating path so as to insert additional loss in the latter path.

The various objects and features of the invention will be better understood from the following detailed description when read in conjunction with the accompanying drawings in which:

Figs. 1 to 7 show schematically telephone'repeater circuitsemploying different modifications of the invention; and

Fig. 5A shows curves illustrating the operation of the modification shown in Fig. 5.

The telephone repeater circuit of Fig. 1 includes a two-wire, one-way transmission path' EA including the one-way amplifier As, for transmitting signals in the direction from west to eastbetween a, west two-way line WL and an east two-way line EL, and a two-wire, one-way transmission path WA including the one-way amplifier Aw, for transmitting telephone signals in the direction from east to west between the same two-way lines. The input of the path EA and the output of the path WAare coupled in conjugate relation with each other and in energy transmitting relation with the two-way line WL by the hybrid coil Hw and associated line balancing network Nw; and the output of the path EA and the input of the path WA are coupled in conjugate relation with each other and in energy transmitting relation with the line EL by the hybrid coil HE and associated line balancing network Nu. Without auxiliary equipment,

proper operation of this circuit requires that the amplifiers An .and Aw.

sum of the losses across the two hybrid coils,

LE+Lw, together with any loss in the conductors must be greater than the sum of the gain in the In accordance with the invention the two thermistors 13,3116. T2 are applied to this circuit.

Each thermistor consists of an element the electhe resistance element. The heating element of the thermistor T1 is bridged across the output of the amplifier Aw in the path WA and its variable resistance element is bridged across the input of the oppositely directed amplifier An in the path EA. The heating element of the thermistor T2 is bridged across the output of the amplifier As: in the path EA and its variable resistance element is bridged across the input of the amplifier Aw in the path WA.

Now, when speech waves from a west subscriber associated with the line WL are impressed on the hybrid coil Hw, they will be divided by it in a predetermined ratio (which is usually 1:1), part going into the path EA directed towards the listener associated with the east line EL and the other part into the path WA directed towards the talker associated with the line WL.

.The portion of the west talkers speech energy passing into the output of the path WA will be dissipated in the output of amplifier Aw. A portion of this energy will be impressed as heating current upon the heating element of the thermistor will be of insuillcient level to raise the temperature of its variable resistance element appreciably, so that the loss in the input of the amplifier As in the path EA remains low. The other portion of the west talkers speech energy passing into thepath EA-will be amplified by the amplifier An and transmitted over that path and line EL to the east subscriber. Since the amplifier As has a high gain, the high amplitude level of the portion of its output applied to the heating. element of the thermistor T2 will heat its variable resistance element sumciently to reduce its resistance to a low value, thereby inserting a large loss into the input of the amplifier Aw in the path WA. Since the magnitude of the echo returnedJzo the west talker through hybrid coil HE and the path WA depends, among other factors, upon the loss Ln across the; hybrid coil He, the "loss inserted into the input of the path WA by thermistor '1: acts as an increase in La and, therefore, decreases the magnitude of this echo. Since the thermistor loss is also in the singing circuit, an alternate use would be to provide a corresponding increase in gain in the amplifier As to reduce the over-all equivalent frorir talker to listener. The echo then would remain the same as before the application of the thermistor.

Similarly, when speech waves from the east talker associated with the line EL are impressed,

upon the hybrid coil HE, they will be divided by it in a predetermined ratio, part going into the path WA directed to the listener and the other part intothe path EA directed toward the talker. The portion of easts speech energy passing into the output of the pathEA will be dissipated in the output of amplifier As. The portion of that energy impressed as heating current upon the heating element of the thermistor T2, if that thermiston is properly designed, will be of insufilcient level to cause an appreciable reduction in the resistance of its variable resistance element so that the loss in the input of the amplifier Aw will re main low. The main portion of the east talker's speech energy in the path EA'will be amplified by amplifier Aw and will pass through hybrid coil Hw and line WL to the east subscriber. Since the amplifier Aw in the path WA has a high, gain, the portion of the east talkers speech energy in the output of that amplifier applied to the heating element of the thermistor T1 will cause sufficient heating of its variable resistance element to decrease the resistance shunting the input of amplifier As to a low value and thus insert a large loss into the input of the path EA.

The circuit arrangement. of Fig. 2 is' similar to that of Fig. 1 except that the hybrid coils Hw and HE and their associated balancing networks Nw and NE have been replaced with three-branch resistance networks RNw and RNE, respectively,

and the delay networks D1 and D2 have been in-.

serted in the input of the paths EA and WA, respectively. Now, when speech waves from the west talker associated with the line WL are impressed on the resistance network Rw, half will then enter the output of the path WA and will be dissipated in the output of the amplifier Aw,

the level of the portion of these speech waves applied to the heating element of thermistor T1 being insufiicient to cause operation of that thermistor to insert loss into the path EA. The speech waves entering the path EA will pass through the delay circuit D1 and will be amplified by the amplifier An. The amplified energy delivered to the resistance network RNE will divide .equallybetween the line EL leading to the east subscriber and the input of the path WA where it passes through the delay circuit D2.

the amplifier An in the path EA will be applied to the heating element of the thermistor T2 heating it sufficiently to cause an appreciable reduc-' tion in the resistance shunting the path WA inthe input of the amplifier Aw, thereby inserting a large loss in that path. Due to the delay in the delay circuit D2, the thermistor T2 acts to place the low resistance across the input of amplifier Aw before the echo waves arrive at that point, thereby effectivel reducing them when they do arrive.

There are two possible ways in which the thermistor T2 or T1 may afiect the circuit operation. If the resistance of T2 or T1 is so low that the lossinserted by it is considerably greater than the sum of the gains of the amplifiers Aw and An,

than the circuit operates essentially as a one-way aera'rce amplifier pointed from talker to listener. If the resistance of T2 or T1 is such that the inserted losses are less than the sum of the two amplifier v bine to appear as a loss in the B circuit. Converting this circuit configuration to that of a feedback amplifier would, of course, require meticulous care in design and would be limited by the performance of thermistors Tl and T2, the impedances looking towards the talker and the listener, and the delay circuits D1 and D2.

Such circuits as that of Fig, 2 would tend to be unstable when not in use. However, there would be a definite limit to the level of oscillations since both thermistors would operate as soon as the level increases sufiiciently. When a signal is applied by the talker this oscillation would be broken since the thermistor T2 or T1 controlled from the output of the amplifier in the talking path goes to a still lower resistance, thereby taking control and conditioning the circuit for transmission in one direction only.

In the arrangements of the invention in Figs. 1 and 2, the thermistors have been so applied as to produce a shunting effect on a voice transmission path when supplied with sufficient heating energy from the other voice transmission path. An equivalent efiect could, of course, be obtained b applying the thermistors at other places in the circuit. For example, as shown in Fig. 3, if the amplifier As in the path EA and the amplifier Aw in the path WA are feedback amplifiers, the variable resistance elements of the thermistors T1 and T2 could be respectivelyinserted in the fl'path of the amplifiers AE and Aw, so as to effectively lower the feedback and thus the gain of the associated amplifier when the thermistor is heated by the speech output of the other armplifier.

Fig. 4 shows a modified arrangement in accordance with the invention applied to a two-way loud-speaking telephone set in which a transmitting circuit TC including the microphone M and the transmitting amplifier Ar, and the receiving reverberation ofthe room in which the loud-,

A portion of the amplified speech waves in the output of speaker and microphone are located. In this circuit, the heating element of the thermistor T1 constitutes a part of the balancing network N and absorbs some predetermined portion of the energy impressed on the hybrid coil H from the transmitting circuit TC, which is ordinarily absorbed by the network N, and the variable resistance element of that thermistor is bridged across the input of the receiving amplifier Aa. The heating element of the thermistor T2 is bridged across the output 'of the amplifier AR which drives the loud speaker LS, and the variable resistance element of, that thermistor is connected across the transmitting circuit TC between i the microphone M and the transmitting amplifier AT. Then, if speech waves are received over the line L andare impressed by hybrid coil H on the receiving circuit RC, the amplified speech waves in the output of the amplifier AR, will heat the thermistor T2 toinsert a loss in the microphone circuit. This, in turn, reduces the amplitude of through hybrid coil H to balancing network N and the associated heating element of the thermistor T1, will activate the latter to insert a large loss in the input of the receiving amplifier AR. thereby effectively reducing the acoustic sidetone, which benefits both the talker at the waves in the output of amplifier AT transmitted fier Aw. This high loss in WA will cause a remicrophone and the listener at the far end of v the line.

Fig. 5 shows a modified arrangement in accordance with the invention operating to provide a differential echo suppressor. Fig. 5 includes a V four-wire repeating circuit like that of Fig. l as indicatedby the use of the same identification characters for corresponding component elements. The heating elements of the thermistors T1 and T2, respectively, constitute part of the line balancing network Nwand of the balancing network NE, and absorb some predetermined portion of the energy ordinarily absorbed by these networks from the transmission received over the paths WA and EA, respectively. The variable resistance elements of the thermistors T1 and T2 are arms of the resistance bridge B whose fixed arms.are the equal resistances R, and which is energized by the battery E1 connected across one diagonal ot the bridge. The winding of a polarized relay P is connected across the other diagonal of the bridge, this relay when operated in one direction causing the operation of the switching. relay S1 and when I operated in the other direction causing the operation of a switching relay S2. The relays S1 and S2, respectively, control the lossvalues of the unbalanced GOO-ohm bridged-T loss networks LN1 including the resistance elements R10 and R11, and LN; including resistance elements R12 and R13, in the output of the amplifier AE in the path EA and in the output of the amplifier Aw in the path WA, respectively. With relays S1 and S2 in their normal unoperated condition, with bridge B balanced, the networks LN1 and LN1; are efi'ectively removed fromv the paths EA and WA, respectively, and the transmission efiiciencies of these paths are high.

Two alternative wiring arrangements are in-' dicated by the character. N and S, respectively. Considering the N wiring, speech waves from a west subscriber incoming'over the line WL pass throughthe hybrid coil Hw and over the path EA to the hybrid coil HE, being amplified by the amplifier AE en route. .A portion of these Waves pass through hybrid coil HE to line EL and over that line to the east subscriber. Another portion of these speech waves pass through the hybrid coil I'IE into the network'NE causing heating of the thermistor T2 to thereby reduce the resistance of its variable resistance portion in bridge B. A portion of the west subscriber's speech waves will be reflected at hybrid coil HE into the path WA and with the amplification providedby the gain setting of the amplifier Aw will pass over'the path WA to'the hybrid coil Hw where part will enter the network Nw and thus supply heating current to the associated thermistor T1 causing the resistance value of T1 to be reduced. Since, however, there is less energy in the thermistor T1 than in the thermistor T2,,by the amount of the return loss at HE, the thermistor T2 will be converted. to a lowerresistance than the thermistor T1. Hence the bridge B will be unbalanced in such manner that the polarized relay P operatesto its right-' hand contact connecting ground to the winding of switching relay S2 and causing the latter relay to be operated from battery E2. The operation of relay S2 will connect resistances including resistances R12 and R13 into the bridged-T loss network LN'z thereby inserting a high loss into the path WA in the output of the ampliduction in the magnitude of the echoes of Wests speech waves returned over that path. When the speech waves received from the west subscriber over the line WL cease, the consequent removal of heating current from-the thermistors T1 and T2 will cause the bridge B to" be balended, and, therefore, the polarized relay P and the switching relay S2 to fall back so as effectively to remove the network LNz from the path When speech waves are received over the line EL from an east subscriber, and at that time the west subscriber associated with the line WL is not talking, part of easts speech waves pass through the hybrid coil H11 into the path WA and pass over that path with the amplification pro- 'vided by the amplifier Awto the hybrid coil Hw.

A portion of these speech waves will pass into the line WL and will pass over that line to the west subscriber. Another portion of these Waves will pass through the hybrid coil ,Hw into the network Nw and will cause heating of the associated thermistor T1 to reduce its resistance. Another porj) tion of the speech waves will be reflected at the hyance of that thermistor to be reduced. Because of p brid coil Hw and, will return as echoes over the path EA with an amplification provided by the amplifier As to the hybrid coil Ha where part of it will enter the network N11 and thus the heating element of the thermistor T2 causing the, resistthe return loss at the hybrid coil Hw, the resistance of thermistor T1 will be lower than that of the thermistor T2, so that the bridge B will be unbalanced in the opposite direction causing the 1 operation of the polarized relay P to its lefthand contact. This will cause ground to be supplied to the winding ofthe switching relay S1 completing an energizing circuitlfor that relay from battery E3. The consequent operation of the relay S1 will connect the resistances R10 and R11 into the bridged-T loss networkLNrthereby placing a loss in the path EA which will reduce the magnitude of 'echo of easts speech currents returned over the path EA. When East ceases talking, the consequent removal of the heating current from the thermistors T1 and T2 will EA bytalking at a level greater than West by the cause the birdge B to be balanced again and relays P and S1 to fall back to efiectively remove the ,loss network LN1 from the path EA.

It will be noted that when West is talking and i has caused Operation of ithe thermistor control arrangement to insert the echo suppressing loss network LNz into the path WA, East can cause the loss to be transferred from the east-to-west transmission path WA to the west-toeast path amount of loss in the network LNz to change the ratio of the resistances of the thermistors T1 and Te so as to unbalance the bridge B in the proper layed for the time required for the thermistor to heat and cool.

If the hybrid coils He and- Hw are remote from,

each other as in the case of a four-wire repeater, wiring S would be employed, in Which case the amplifiers An'and Aw are assumed close to each other. The action of the thermistors T1 and T2 in the case of S wiring is similar to that just described for the N wiring except that the heat-. ing energy for the two thermistors is then derived directly from the outputs of the amplifier in the path WA and EA, respectively, at a point in front I, of the inserted loss pads LNz and LNI instead of from the net side of hybrid coils. In this case, when the relay S2 or $1 operates in response to operation of the polarized relay P to its righthand or left-hand contact, it locks up through a back contact of the other switching relay S1 or S2, thereby keeping the loss in the path WA or EA until such time as East or West, respectively, talks.

plifiers and ahead of the inserted loss, and since these are equal level points in the circuit, East can always transfer the loss by talking at a higher level than West.

,In the difierential action of the thermistors T2 andTi, there are two separate actions possible with the S wiring. One is the diiferential action due to the return loss across th hybrid coil and the second is the time differential due to the Since the thermistors derive their heating energy directly from the outputs of the am-' aera'ree ance element of the thermistor T1 having its I heating element associated with the network Nw,

when the west subscriber talks first, or talks louder than theeast subscriber by an amount equal to the loss value of the resistance loss network LNi. -This will cause the switching relay S2 to be operated to effectively insert the full loss value of the loss network LNz into the path WA to suppress the echoes of Wests speech waves and to give directional control of the circuit to West. Similarly, the operation of the polarized relay P1 to its left-hand contact when the biasing transit time from amplifier AE through hybrid coil HE and back to amplifier Aw or'from amplifier Aw through hybrid coil HE and back to am-' plifier Air. Now, if the level is the same at the two points there will still be a resistance differential AR during the heating time of th thermistors due to the time difierential At. This is shown by the curves of Fig. 5A in which the resistance variations of the two thermistors are plotted against time. On cooling of the thermistors, the action would be in the reverse direction and might be considered in the nature of an anticipatory action. If this were undesirable, relays S1 and S2 could'be made to include a slowoperate or release arrangement, in which case the bridge B will return to balance on cooling without transferring the network loss. Ordinarily, this latter feature of time differential would be overshadowed by the level differential.

The circuit arrangement of 6 is similar to that of Fig. 5 in every respect except for the type of differential action. In Fig. 6, the variable resistance elements of the two thermistors T1 and r T2 are inserted in series respectively with the two ,winding from the associated batteries 'due to a v greater reduction in the resistance of the variable resistance element of the thermistor T2 having its heating element associated with network NE, than in the resistance of the variable resist-.

current supplied to the upper winding of that relay is greater than that'sup plied to the lower winding of the relay due to a greater reduction in the resistance of the variable resistance element of the thermistor T1 than in the resistance of the variable resistance element of the thermistor T2, when East starts talking firstzor talks louder than West by an amount at least equal to the loss value of the loss network LNz in the path WA, will cause the operation of the relay S1 to insert the full loss value ofthe loss network LN1 into the path EA to suppress echoes of Easts speechwaves and to give him directional control of the circuit.

v Fig. 7 shows a modified arrangement of the invention which is essentially similar to that shown in Fig. 2 except that relays controlled by the,

variation of the resistance value of the variable resistance elements of the" thermistors T1 and T2 are used to insert the required echo suppression loss into the paths EA and WA, respectively.

The relays R1, R2 R1; are marginal relays .of different sensitivities having their upper (biasing) windings supplied in series with biasing current from battery E4 and theirlower' (operating) windings supplied in series with the variable resistance element of thermistor Tzwith operating current also from the battery E4, the heating element of the thermistor T2 beingconnected across the output of the amplifier As in the westto-east transmission path EA. Similarly, relays R11, R12 RlK are marginalrelays of difierent sensitivities having their lower (biasing) windings supplied in series ,with biasing current fromthe battery E5 and their upper (operating) windings supplied. in series with the variable resistance element of the thermistor T1 with operating current also from the battery E5,

the heating element of the thermistor T1 being bridged across theoutput of the amplifier Aw in the east-to-west transmission path WA. The

relays R1, R2,.Rx 'are arranged not to operate with the normal bias, but when the resistance of thermistor T2 is reduced in response to the output waves of the amplifier AE applied to its heating element, when signal transmission is taking place over the path EA, the increased oper'aterate to insert the loss L2 intothe path WA in front of amplifier Aw, and still further reduction in the resistance of the variable resistance element of thermistor T2 in response to higher level speech waves applied to its heating element will cause the least sensitiverelay Rx to operate to insert the additional loss Ls into the path -Ti will .cause the reduction of its resistance to operateflrelays R11, R12 Rn; in turn as the aavavee ance devices the resistances of which var non linearly with'temperature, means responsive to the amplified signal output of the amplifying means in each of said paths to heat a different one of said variable resistance devices to a temperature which will produce a given change in the resistance thereof, and means responsive to said given change in the resistance of a variable level of the speech current increases to insert the losses L11, L12 Ilia in the path EA in the input of the amplifier Am, the total loss inserted being proportional to the level of the eastto-west speech current flowing over the path WA. The loss elements Li, L2 LK and L11, L12 Lu: maybe elements of bridged-T loss networks such as illustrated in Figs. 6 and 7.

As described in connection with the similar repeater of Fig. 2 employing three-branch resistance coupling networks RNw and RNE and delay circuits Diand D2, the repeater circuit of Fig. 7 when not in use for transmitting speech waves would be in the condition of stabilized oscillation which would cause certain of the relays R1 RK and R11 Rm to be operated. The transmission ofspeech waves in the direction from west to east over the path EA will cause the thermistor T2 to go to a lower resistance and, therefore, will cause more loss to be inserted in the oppositely directed repeating path WA. The consequent reduced output of the amplifier Aw in the path WA would in turn cause certain relays in the R11 Rm chain to fall back removing loss from the path EA and pointing transmission from west to east. The action will be similar in case the eastsubscrlber talks first, the relay chain R11 R11; operating to insert the required echo suppression loss in the path EA and the consequent reduced output of the amplifier Aacausing certain relays inthe chain R1 Rx to fall back to remove loss from the path WA and pointing transmission from east to west. By suitably choosing the values of the losses L1 LK and Lu LlK, the repeater circuit can be made to cease oscillating and become a stable amplifier pointing in one direction at a time when speech is applied. When no speech is applied the circuit returns to oscillation which-is not necessarily a severe penalty if the level and frequency are controlled.

In the practical application of the schemes discussed above in connection with Figs. 1, 3 and 4, the variable resistance elements of the thermistors might not be placed directly across the voice resistance device in response to the amplified signal output of the amplifying mean in one path when it is transmitting signals, to efiectively iiiv sert an appreciable loss in the other path.

2. The system of claim 1,-in which each of said variable resistance devices is a thermistor hav ing a variable resistance elementand a heating element therefor, the heating element of each thermistor being connected across the output of the amplifying means in a difierent one of said pat and the variable resistance element thereof being associated With'the other path so as to insert said appreciable loss therein in-response to said given change in its resistance.

3. The system of claim 1, in which each of said variable resistance device's is a thermistor having a variable resistance element the resistance of which decreases with temperature, and a heating element therefor, the heating element of each thermistor being connected to the output of the amplifying means in a different one of said paths so as to be heated in accordance with the ampli tude level of the amplified signals in that path, and the variable resistance element of that thermistor being connected effectively in shunt I with the other path in front of the amplifying means thereon, each thermistor being such that its resistance value will be substantially unafiected by waves applied to the heating element of a level corresponding to signal echoes. V

4. The system of claim 1, in which each of said amplifying means comprises a feedback amplifier and each said variable resistance device is a thermistor having a. variable resistance element and a heating element therefor, the heating ele-.

ment of each thermistor being connected across the output of the amplifying means in a different one of said paths so as to be heated by the amplified signal output thereof, and the variable resistance element thereof being connected in the feedback path in the amplifier in the ;other path transmission paths as illustrated because of the undesirable change in impedance which it would bring about. Preferably, the variable resistance elements of the thermistors would be placed in series with a marginal relay which would inturn pendently. Various other modifications of thecircuits illustrated and described which arewithin the spirit and scope of the invention will occur to-those skilled in the art. ,7

' What is claimed is: 6

. W 1 In a two-way telephone system including two oppositely directed one-way transmission paths for the telephonic signals in opposite direc'- tions, each including amplifying means, means to prevent singing and suppress echoes in said system during signal transmission thereover in either direction, comprising two variable resistpaths, each including in such a manner. as to reduce the amplifier gain in proportion to the change in its resistance when heated above'a given temperature.

5. In combination, transmission system, two two-way lines; two oppositely directed one-way signal transmission necting said lines, and means to prevent singing and to suppress echoes insaid system during signal transmission in either. direction thereover, comprising two 'thermistors each consisting of a variable resistance element theresistance of which decreases rapidly when its temperature is increased above a given value, and a heating element therefor electrically isolated from the vari-- able resistance element, the heating element of each thermistor being bridged across the output of the amplifying means in a different one of said paths, and itsvariable resistance element being associated with the other path in such a manner that when its temperature is increased above said given value an appreciable loss is inserted in said other path, the thermistors being proportioned "and the gain of the amplifying means in said paths being selected so that the level of amplified signals a plied to a heating in a two-way telephone an amplifying means, conelement of. a ther-' mistor whena path is transmitting signal is sufficient to raise the temperature of its associated variable resistance element above said given value but the level of amplified signal echoes in the non-transmitting path applied to a heating element of a thermistor is insuficient to raise the temperature of the variable resistance element thereof to said given value.

6. The system of claim 5, in which the variable resistance element of each thermistor is placed in series with the operating winding of a relay in a direct current circuit, the increase in current flow in said winding with the reduction in resistance of said variable resistance element when it is heated to said given temperature in response to the amplified signal output of the amplifying means in one path causing operation of said relay to insert said appreciable loss in said other path.

awe res tially reduced with respect to that of the variable resistance in the other arm when one path is transmitting direct signals and the other is not, causes said appreciable loss to be inserted in said other path.

8. The system of claim 5, in which said transmission paths are coupled in conjugate relation '7. The system of claim 5, in which the variable 20 anced bridge circuit, and the unbalance of said bridge circuit when the resistance of the variable resistance element in one arm is'substanwith each other and in energy transmitting relation with one of said two-way lines by a hybrid coil and associated line balancing network, the heating current supplied to the heating element of each thermistor being derived from the signal current fed into a different one of the networks when the path connected thereto is transmitting direct signals, and the variable resistance elements of the two thermistors are connected in a diiferential circuit, the unbalance in said differential circuit when one of said variable impedance elements is heated above said given temperature and the other below said given temprature in response to direct signal transmission in one path only causing saidappreciable loss to be inserted in said other path.

KENNETH G. VAN WYNEN. 

